1. Field of the Invention
The present invention generally relates to flip-chip packaging, and more particularly to a method of forming metallic bumps on the I/O pads of a semiconductor device without under bump metallurgy and a metallic bump structure thus formed.
2. The Prior Arts
Flip-chip packaging utilizes bumps to establish electrical contact between the I/O pads of a semiconductor die (e.g., a chip) and the substrate or lead frame of the package. Conventionally, there is a so-called under bump metallurgy (UBM) located between the bump and an I/O pad of the semiconductor die.
An UBM generally contains an adhesion layer, a barrier layer and a wetting layer, arranged in this order on top of an I/O pad. The bumps themselves, based on the material used, could be classified as solder bumps, gold bumps, copper pillar bumps and bumps with mixed metals.
To form bumps on the UBMs, usually a technique such as electroplating, printing, or stud bonding is used. For electroplating, patterned plating resists are first formed on the UBMs and then metals are plated. For printing, solders are first printed on the UBMs and the solders are thermally cured into the bumps. For stud bonding, it is used for limited gold bumping only. The semiconductor die with bumps is then singulated from the semiconductor wafer, and soldered onto a substrate or lead frame.
UBM not only prevents the copper trace on the semiconductor die from dissolution into solder but also functions as a conducting plate if electroplating is the means of forming metallic bump. Besides, the wetting layer of the UBM provides reliable solderability for forming solder bump, if aluminum is used in the I/O pad.